1. Field of the Invention
The present invention relates to a device for controlling a hydraulic pump provided in a construction machine such as a hydraulic excavator.
2. Description of the Background Art
The background art of the invention will be explained by describing a hydraulic excavator. A general hydraulic excavator includes a crawler-type lower propelling body, and an upper slewing body loaded on the lower propelling body slewably about an axis normal to the ground, the upper slewing body attached with a working attachment. The attachment includes a boom, an arm, a bucket, a boom cylinder for raising and lowering the boom, an arm cylinder for pivotally moving the arm, and a bucket cylinder for pivotally moving the bucket. The hydraulic excavator further includes, as hydraulic actuators other than the above cylinders, left and right travelling motors for travelling the lower propelling body and a slewing motor for slewing the upper slewing body.
In such a hydraulic excavator, there is performed a horsepower control of controlling a discharge amount (pump discharge amount) of a hydraulic pump in accordance with a pump pressure i.e. a load pressure of the hydraulic pump. Generally, as a characteristic for the horsepower control for constant horsepower control of keeping the horsepower constant, set is a P-Q diagram as shown in FIG. 7, based on which a pump flow rate in accordance with the pump pressure is determined.
However, for the hydraulic excavator, it is not absolutely preferable to perform the control of always keeping the horsepower constant. Specifically, the travelling operation in a high pressure region requires a greater horsepower than the operation for work e.g. excavation by the actuators other than the travelling motors, while a working operation is performed in a low-pressure region or in an intermediate-pressure region (e.g. from 10 MPa to 20 MPa), seldom in a high-pressure region. Hence, setting the P-Q diagram representing constant horsepower control as a horsepower characteristic on the basis of the horsepower during a travelling operation time may cause energy loss during the working operation time, which resultantly lowers the fuel consumption rate; on the contrary, determining the horsepower on the basis of the horsepower during a working operation time may cause a horsepower shortage during a travelling operation time, particularly when the machine travelling on an uphill or travelling on a slope.
Japanese Utility Model Unexamined Publication No. SHO 62-134902 discloses a technology for solving the above problem. As shown in FIG. 8, the technology includes: setting in advance two types of characteristics which differ from each other in the horsepower substantially throughout the entire pump pressure range, namely, a characteristic A that gives a horsepower relatively large as a whole and a characteristic B that gives a horsepower relatively small as a whole; and selecting the characteristic A during a travelling operation time and the characteristic B during a working operation time, respectively. The technology lowers the horsepower as a whole during the working operation time to suppress energy loss, thereby improving the fuel consumption rate. However, this involves a harmful effect of a lowed flow rate in the low-pressure region and in the intermediate-pressure region, which are practical regions, the lowered flow rate delaying the cycle time for a working operation to thus lower the working efficiency.